Types of Friction, Discovery of wheel

1. FRICTION (Part-2)
CLASS:VIII
SUBJECT:PHYSICS

2. (Please watch the videos before going through this presentation)
TYPES OF FRICTION
Static Friction Sliding Friction Rolling Friction

3.  Static Friction is the friction which exists when there is no relative motion between the two surfaces in
contact.
 When an external force acts on a stationary box, static friction tries to oppose the motion of the box.
When a force ’F’ is applied to move a large
box, the box remains at rest. This is because,
the static friction (F) balances the applied
force by opposing the motion of the box.
When applied force is increased to ‘2F’,
static friction also increases to ‘2F’
opposing the motion of box. Thus , again
the box remains a rest.
 Thus , static friction is a SELF - ADJUSTING force but can increase only upto a certain
limit.

4. LIMITING FRICTION
 The maximum value of static friction is called limiting friction.
 When the applied force is increased beyond the limiting friction, the
body begins to slide over the surface on which it was resting.
Ans * When the board just begins to slide, the force
of friction is Static friction having its maximum value
(Limiting friction ).
* The board still keeps on sliding even if we remove
the marbles .The friction that exists now, is Sliding
friction.
This means that the once the board starts moving ,
the friction becomes lesser.
Q. Once the board begins to slide, what
will happen if we remove the weights one
by one from the pan ?

5. Sliding Friction comes into play whenever an object moves along or slides over
the surface of another object.

6. Sliding friction is less than the Static friction between the same
two surfaces.
POSSIBLE EXPLANATION:
• * Motion starts when the inter- locking of irregularities
(ups / downs) get UNLOCKED.
* Once the motion starts the ups / downs do not get
enough time to get interlocked .
* So, its easier to keep on moving an object than to
start it from rest.

7. Rolling friction is the force of friction that comes into play when
one body rolls over the surface of another body.
Static friction > Sliding friction > Rolling friction

9. • The fact that Rolling friction is less than the Sliding friction, led to the discovery of wheels.
• It is much easier to carry a heavy load on a trolley with wheels than to push it.
• Wheels are used extensively in our daily lives for transportation. They save labour and energy
and this is all because of the reduced rolling friction.

12. EXERCISE QUESTIONS:
PART D
Q1. Define the terms : a) static friction b) sliding friction
c) rolling friction
Answer: Refer slides- 3, 5 and 7.
Q4.

13. a) Give the (likely) value of the force of limiting friction.
Answer: 3F units
b) Why does the box stay at rest in fig (a) and fig (b) ?
Answer: This is because the applied force ( F) is balanced by the static friction
acting in the opposite direction. (Static friction adjusts itself according to the applied
force)
c) Mark the direction of the frictional force and mention the kind of frictional force
that comes into play.
Answer: The direction of frictional force in all the three cases will be opposite ( )
to the direction of Applied force.
In (a)- Static friction (b)- Static friction (c)- Limiting friction
Q5. Name the kind of frictional force that comes into play when a book, kept on a
collection of pencils, is moved by pushing it.
Answer: Rolling friction

14. Answer: In case ‘C’, the ball will cover the longest distance because of the smooth
surface, friction will be minimum in this case.
PART E

15. Q3. Explain why sliding friction is little less than static friction ?
Answer: The friction between two surfaces is because of the interlocking of irregularities of
the two surfaces.
During sliding, contact points do not get enough time to get interlocked. Therefore, less
friction.
But, when the object is at rest, surfaces interlock well, therefore more friction.
This explains why sliding friction is little less than static friction.
Q4. Give reasons for the following:
c) The force, needed to start a cart is greater than the force needed to keep it moving with a uniform
speed.
Answer: * When the cart is at rest, surfaces are interlocked well. So, friction will be more.
* But once the motion starts, the inter- locking of irregularities get UNLOCKED.
* So, when the cart is moving, the irregularities do not get enough time to get interlocked.
Therefore, friction will be less.
(Check the given links)
https://www.youtube.com/watch?v=9SMp-jnh8lg
https://www.youtube.com/watch?v=HRe90ySP38U